If you still want to do the leveling job, find a strong point in the stand and place a car jack underneath it and lift it just enough to place the shimmers.
You'll need one people to lift the jack and another to place the shims. If the lifting spot is sturdy enough, you could even lift the tank with some water in it. Maybe leaving a few inches will keep the fishes fine till the work is done. Just make sure the floor can stand the jack.